Journal of Waterway, Port, Coastal, and Ocean Engineering

Permeable pile groins have been built on the southern shores of the Baltic Sea in large numbers for a century and a half, yet no explanation has been found in the literature on how these function. It is shown that pile groins act as a hydraulic roughness on the longshore current and that their effect on waves is negligible. By reducing, but not blocking, the littoral current, the velocity differential between the velocity seaward and in the pile-groin fields is smaller than with impervious groins. Hence, the circulation in the groin field due to the fluid drag at the seaward boundary is weaker and is concentrated mainly in the velocity interface in the form of small eddies. The reduced littoral current velocity in the pile-groin fields leads to a reduction of turbulence produced at the bed by the wave-current interaction, to a reduced amount of sediment suspended, and to a thinner layer of suspended sediment. Hence, the amounts of sediment advected by superimposed currents are reduced. The reduction is most effective for erosive-wave conditions and enables trapping or holding of sediment until the continuity of littoral transport is reestablished and the cross-shore transports balance over time.

There has been renewed interest in the use of beach dewatering as a technique to combat beach erosion. Although several test and prototype installations have been undertaken worldwide, no guidelines are available to aid the engineering design of such projects. In this paper, we present a numerical model that simulates two-dimensional ground water flow in a coastal aquifer under beach drainage, including water table lowering and seepage face reduction. The boundary element method is applied to solve the Laplace equation for the velocity potential. An internal boundary is used to simulate drainage. We conducted simulations for two different types of drainage systems, i.e., artificial and gravity drainage. The behavior of the water table from the simulations was found to be consistent with field observations. The results also showed that the location and size of the drain affect the efficiency of the drainage system. The system efficiency slightly increases with the size and vertical elevation of the drain. As the drain is shifted landward, the system becomes considerably less efficient. Although they do not represent design criteria, these results indicate considerations which affect the performance of beach dewatering system.

Design formulas for placed block revetments are evaluated. The various formulas, as published in literature, have a different background or are based on different experiments. Most formulas are based on an analytical solution of a ground-water flow equation for the flow in the filter layer and a schematized pressure distribution on the slope due to wave attack. Empirical formulas describing the wave pressure distribution as a function of wave height, wave steepness, and slope are presented. The pressure distribution used was based on experiments with regular waves. Recent experiments provide the opportunity to improve the wave pressure distribution, using the results of irregular waves. The resulting design formulas are described. These design formulas are based on laminar flow. The influence of turbulent flow is investigated. The formulas are used in a calculation example. It is shown that the permeabilities of cover layer and filter layer are very important for the maximum wave height the revetment can withstand.

An analytical solution is developed to investigate the behavior of a submerged or surface-piercing, long tethered breakwater of rectangular cross section. The equations of motion are solved to provide the surge, heave, and pitch responses of the structure. The wave reflection and transmission coefficients are calculated. Numerical results are presented for a range of wave and geometric parameters that illustrate the suitability of this type of structure as a barrier to wave action. Small-scale physical model tests were carried out to verify the theory. Reasonable agreement is found between theoretical and experimental values of the reflection and transmission coefficients. It was observed both in the numerical results and model tests that low wave transmission coefficients occur when the diffracted and radiated waves in the lee of the structure are of similar magnitude, but 180° out of phase. The lowest transmission coefficients occurred near the surge natural frequency of the floating breakwater, where significant energy dissipation was present. Under these circumstances, destructive wave interference and energy dissipation combine to give transmission coefficients on the order of 0.5.

Laboratory data on sand ripple spacing due to combined wave-current motion are collected to examine the validity of the empirical formula proposed by Tanaka and Shuto (1984). Data from Gray et al. (1991) and from Van Rijn et al. (1993) showed considerable discrepancy from the formula. The discrepancy can partly be attributed to insufficient consideration of the grain size effect in the previous study. Furthermore, incorrect evaluation of the orbital diameter due to wave motion, which was pointed out by Christoffersen and Jonsson (1985), can be another reason for the discrepancy. A new formula for the spacing of wave-current-induced sand ripples is derived. The agreement between the measurement and the prediction can be much improved as compared with the previous formula. The full-range equation for the wave-current friction coefficient proposed by Tanaka and Thu (1994) is applied in the formulation of steepness of sand ripples. Empirical equations for pure wave ripples can tentatively be used for estimating the steepness of sand waves.